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  1. #1
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    Heat load calculations Help!



    Ok im having a few dramas calculating airchange load. Product load & defrost load. Here is what i have so far. Im calculating over 24 hours.

    The room is loaded with 1500kg of product per day
    The storage temp is -35
    Temp of product entering the room is 35 degrees
    The specific heat of the product above freezing is 3.182kj
    The specific heat of the product below freezing is 1.717kj
    The freezing temp is is -1.17
    The latent heat of fusion is 233.48kj
    there are 6 x 20 minute defrosts per day with the heater elements totaling 10Kw
    Ineed to allow for 2 complete air changes

    here is what i have so far

    Airchange Load = IRVxACFxenergyxusage/time

    IRV -LxWxH = 15x10x5 = 750m3
    Now i have not been given an ACF(or is that 2^ )
    Energy - 60%R.H, - 35 degrees storage =0.1045
    Usage Factor - im assuming it is 1
    1 hour = 3600 seconds x 24 = 86400 seconds

    So i have gone :

    IRVxACFxEnergyxUsage/time

    750m3 x (2?) x 0.1045 x 1/86400= 1.814236111Kw

    Now for defrost load

    10Kw x 2/24=0.83Kw

    Now for the product Load

    Q=m x cp x deltaT

    m=1500kg
    cp=3.182 - Above freezing
    1.717 - below freezing

    delta T - 35+1.17=36.17 - Delta T above freezing
    -35--1.17=33.83 Delta T below freezing

    Pull down time 24hours =86400 seconds

    so 1500kg x 3.182 x 36.17=172.639/86400=1.998Kw
    1500kg x 1.717 x 33.83=87.129/86400=1.008Kw

    1.998 + 1.008=3.006Kw

    how do i work out the latent heat?

    Thanks for your help here guys. I cant be to far off can i?



  2. #2
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    Re: Heat load calculations Help!

    anyone?

  3. #3
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    Re: Heat load calculations Help!

    Quote Originally Posted by jbomba View Post
    anyone?
    Google and locate "Coolpack" its a free download and has all the information you need.

  4. #4
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    Re: Heat load calculations Help!

    1500* 233= 359500Kj/3600=97Kwhrs
    97/ 24=4.045Kw. plus all others.
    But I would not use the total rating as the rating for your condensing unit.
    (Total *24)/18 run hours.
    Other issues that need to be considered is this just freezing product or is it also longer term storage.
    You need to to size your condensor for a higher evap temp (SST) and/or install CPR.
    There are also limits on the product
    If it is one big lump then will not freeze in 24hrs, but if very small bit will freeze easy, raising room temp.

  5. #5
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    Re: Heat load calculations Help!

    Quote Originally Posted by B G Scott View Post
    Google and locate "Coolpack" its a free download and has all the information you need.
    this is just an exercise im doing at TAFE so i just wanna get my head around the actual calculations. Any help would be greatly appreciated

  6. #6
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    Re: Heat load calculations Help!

    can anyone help me nail this one?

  7. #7
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    Re: Heat load calculations Help!

    Hi there,

    Latent heat unit is kJ/kg NOT kJ.

    So : m x LH /86400 = 1500 x 233.48 /86400 = 4.05 kW

    What seems to be the problem?

    Cheers
    Even Einstein Asked Questions

  8. #8
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    Re: Heat load calculations Help!

    May be your are not understanding Kj/Kg this is energy, there is no time involved, so you have to work out total energy then divide by a time you want this to happen in. But you then have to include defrost and off times, which increases the refrigeration capacity over that of your original calculation

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